1. Field of the Invention
The present invention relates to a crystallization apparatus which crystallizes a non-crystal semiconductor film with irradiating laser beam to a film made of polycrystal semiconductor or an amorphous semiconductor, a phase modulation element used in this crystallization apparatus, a crystallization method, and a device. In particular, the present invention relates to a crystallization apparatus which crystallizes a non-crystal semiconductor film, such as a polycrystal semiconductor or an amorphous semiconductor, with irradiating laser beam having a predetermined light intensity distribution subjected to phase modulation by using a phase modulation element to a film made of polycrystal semiconductor or an amorphous semiconductor, a phase modulation element used in this crystallization apparatus, a crystallization method, and a device.
2. Description of the Related Art
Conventionally, a thin film transistor (TFT) is used for, e.g., a switching element which controls a voltage applied to pixels in, e.g., a liquid crystal display (LCD). This thin film transistor is formed on an amorphous silicon layer or a polysilicon layer.
The polysilicon layer has a higher mobility of electrons and holes than that of the amorphous silicon layer. Therefore, when a transistor is formed on the polysilicon layer, the switching speed is increased as compared with a case that a transistor is formed on the amorphous silicon layer. Further, the response speed of a display is improved. Further, a peripheral LSI can be constituted of thin film transistors. Furthermore, there is an advantage that the design margin of any other component can be reduced. Moreover, when peripheral circuits such as a driver circuit or a DAC are incorporated in a display, these peripheral circuits can be operated at a higher speed.
Although the polycrystal silicon is formed of an aggregation of crystal grains, it has a lower mobility of electrons and holes than that of single-crystal silicon. Additionally, many thin film transistors formed on the polycrystal silicon have a problem of irregularities in crystal grain boundary number in a channel portion. Thus, in order to improve the mobility of electrons and holes and reduce irregularities in crystal grain boundary number in the channel portion, there has been proposed a crystallization method which generates crystallized silicon with a large grain size.
Conventionally, as this type of crystallization method, there is known a phase control excimer laser annealing (ELA) method which generates a crystallized semiconductor film by irradiating a phase shifter approximated to a polycrystal semiconductor film or an amorphous semiconductor film in parallel with an excimer laser beam. The detail of the phase control ELA method is disclosed in, e.g., “Journal of the Surface Science Society of Japan, Vol. 21, No. 5, pp. 278–287, 2000”.
In the phase control ELA method, a light intensity distribution with an inverse peak pattern (pattern that a light intensity is minimum at the center and the light intensity is suddenly increased toward the periphery) that a light intensity is lower at a point corresponding to a phase shift portion of a phase shifter as compared with the periphery is generated. Further, a polycrystal semiconductor film or an amorphous semiconductor film is irradiated with a light beam having this light intensity distribution with the inverse peak pattern. As a result, a temperature gradient is generated in a melting area in accordance with the light intensity distribution. A crystal nucleus is formed at a part which is solidified first or which is not melted in accordance with a point where the light intensity is minimum. A crystal grows in the lateral direction from the crystal nucleus toward the periphery (which will be referred to as a “lateral growth” or a “lateral-directional growth” hereinafter), thereby producing single crystal grains with a large grain size.
Conventionally, in “Electrochemical Society Proceeding Volume 2000-31, page 148–154, 261–268”, for example, an element having a pattern which forms a V-shaped light intensity gradient distribution (which is determined as a mask #2) and an element having a pattern which forms a light intensity minimum distribution with an inverse peak shape (which is determined as a mask #1) are both realized by providing phase steps on a substrate of SiO2. Furthermore, a non-crystal semiconductor film is crystallized on a substrate (a substrate to be processed) with irradiating an excimer laser beam in a state that the two elements are in closer vicinity to the substrate.
Therefore, at a crystallization step, when an ablation phenomenon is generated from a substrate, an evaporant adheres to an opposed surface of the mask 1 to which the substrate is opposed. At the next step after the evaporant has adhered, irregularities are produced in a light intensity distribution formed by the mask #1 and the mask #2, and a crystallized area and a crystallized shape become disordered. Moreover, in regard to application of a laser beam with the mask 1 and mask 2 being in close vicinity to the substrate, an operation which always maintains a distance of several μm order between the mask #1 and the substrate constant requires a long time, thereby relatively delaying a tact time.
Additionally, in “IEICE (The Institute of Electronics, Information and Communication) transactions Vol. J85-C, No. 8, p. 624–629, August 2002”, an element having a pattern which forms, e.g., a V-shaped light intensity gradient distribution is realized by a thickness distribution of a light absorption material SiONx, and an element having a pattern which forms a light intensity minimum distribution with an inverse peak shape is realized by phase steps of SiO2. However, these two elements are laminated and formed on one substrate. Further, a crystallized semiconductor film is generated on a substrate with irradiating an excimer laser beam in a state that the substrate is in close vicinity to this one element substrate.
Furthermore, in Jpn. Pat. Appln. KOKAI Publication No. 2000-306859, an image formation optical system is arranged between a phase shifter having a line-and-space pattern whose phase difference is 180 degrees and a substrate. Moreover, a crystallized semiconductor film is generated on the substrate with irradiating a light beam having a light intensity distribution with an inverse peak pattern generated through the phase shifter to the substrate through the image formation optical system.
However, in the conventional technique (proximity method) by which a substrate is in close proximity to an element, the element is contaminated due to ablation in a semiconductor film, which obstructs the excellent crystallization. Additionally, the substrate and the element must be separated from each other every time processing moves to another processing area on the substrate, which prolongs the processing time. Further, since a gap which should be set between the element and the substrate is very small, a detection light beam for position detection is hard to be led into this narrow light path, and a gap adjustment is difficult.
On the other hand, in the conventional technique using a phase shifter having a line-and-space pattern whose phase difference is 180 degrees, a trough portion in a formed light intensity distribution with an inverse peak pattern becomes too deep. In this case, since a crystal does not grow unless a light intensity is not less than a predetermined threshold value, an uncrystallized area becomes too large, and a crystallized semiconductor film with a large grain size cannot be generated. Furthermore, it is impossible to obtain a large gradient of a light intensity distribution pattern with an inverse peak pattern in order to perform crystallization with a large grain size.